Corrosion scientists have often found the need to perform concurrent testing of large numbers of materials, at a variety of set potentials, all within a common test tank. A system was therefore required to provide the potentiostatic control functions and the high accuracy measurement capabilities required to fulfill many of todays corrosion testing needs.
The potentiostat is an instrument that automatically varies the current flow between a test electrode and an auxiliary electrode in order to regulate the potential between the test electrode and a reference electrode.
In a typical three-electrode potentiostatic circuit, the potentiostat has two inputs and one output. The inputs are potentials from the reference electrode and a user-adjusted set potential. The reference electrode is buffered so that negligible current flows through it. The potentiostat causes the potential between the reference electrode and the test electrode to equal the set potential by controlling the current flowing between the auxiliary electrode and test electrode. The value of the current is determined by some non-intrusive method of current measurement. Commercial corrosion test systems, such as the Princeton Applied Research Model 350A, are capable of very accurately controlling and measuring potentiostatic reactions over an extremely wide range of potentials and currents. However, the systems are quite expensive and are only capable of controlling a single three-electrode system at one time.
Another requirement of corrosion scientists was the need to perform concurrent testing of the cathodic properties of a large number of materials at several different potentials. The same test tank was to be used to ensure that all materials experience the same electrolytic conditions. Since a single test could considerably run for months or even years, it was uneconomical to use commercial three-electrode test systems to test a large number of materials. Therefore, a multiple-test electrode, multiple-set-potential instrument, dubbed the Long-Term Current Demand Control System, was designed and built to fulfill the cathodic research requirements of corrosion scientists. Such a control system is disclosed and claimed in Canadian Pat. No. 1,193,659 issued Sept. 17, 1985 in the name of Her Majesty the Queen in Right of Canada as represented by the Minister of National Defence.
Another need which has arisen for corrosion scientists is to perform corrosion studies on the anodic side of the polarization characteristics of several materials. It is in this region that pitting and crevice corrosion takes place. A practical use of this knowledge is in the consideration of possible materials for undersea applications. An example might be the examination of new alloys for use in the construction of variable-depth sonar cables. Anodic protection is also used quite widely in industry; e.g. to inhibit corrosion in the chemical vats used at pulp and paper mills.
Although it may appear that the determination of the polarization characteristics of materials should not be difficult, there is an induction time during which, for any given set potential, the material must undergo a transient chemical reaction before settling down to its steady-state condition. A material may require an induction time of days or even weeks before its ultimate current value is reached. Furthermore, because of the manner in which pits and crevices form in corroding materials, most of the change in the magnitude of the current flow might take place in the last few hours or days of the test. Therefore, accurately establishing the polarization curve of a single material could require months of testing if a reasonable number of points are measured and if only a single potentiostat is available.
To resolve the problem would require the use of a bank of potentiostats controlling a group of identical test specimens, with each potentiostat determining a single point on the material's polarization curve. As indicated earlier, it would be economically unreasonable to use a large number of commercial potentiostats to perform such testing. Therefore, it was recognized that there was a distinct need for an anodic or positive-potential potentiostatic control system.
As a result, the General Purpose Potentiostatic Test System (GPPTS), which is the subject of this invention was designed. The GPPTS is an electronic instrument which is capable of potentiostatically controlling a large number of test electrodes at several different set potentials on both the anodic and cathodic sides of their polarization characteristics.
For example, the GPPTS can control up to 56 test electrodes, in groups of seven, at up to eight different set potentials. The set potentials can be adjusted to any value between .+-.2.0 volts with respect to a reference electrode. Furthermore, the entire test can be performed within a single large test tank using a single auxiliary electrode. The GPPTS is provided with a built-in measurement system which can accurately determine the impressed current at any test electrode and display it with 31/2 digits of resolution.
In the new design, the auxiliary electrode is grounded and the test electrode is directly controlled by the potentiostat.
The current measurement takes place at the test electrode. In a single-test-electrode system, the current measurement could also take place at the auxiliary electrode. However, since it is desired to extend the design to a multiple-test-electrode--single-auxiliary-electrode system, it is necessary to make the measurement at the test electrode.
It is therefore an object of the present invention to provide a bipolar potentiostatic test system which is capable of potentiostatically controlling a large number of test electrodes at several different set potentials on both the anodic and cathodic sides of their polarization characteristics.
Another object of the present invention is to provide a bipolar potentiostatic test system having a switching and measurement system which will enable the rapid and precise determination of set potentials as well as any individual test electrode current.
Yet another object of the present invention is to provide a bipolar potentiostatic test system in which all test electrodes are placed in the same test tank and will draw current directly from the same auxiliary electrode.
According to one aspect of the invention, there is provided an apparatus to provide bipolar potentiostatic control of test electrodes, said test electrodes forming part of an electrical potentiostatic circuit having an auxiliary electrode and reference electrode wherein said electrodes are immersed in an electrolyte, comprising potential supply means for providing an adjustable set potential to said potentiostatic circuit; buffer means connected to said reference electrode to provide a buffered signal; adding means for adding said set potential and said buffered signal to obtain a computed potential; driver means for maintaining said test electrodes at said computed potential by controlling the current flow between said test electrodes and said auxiliary electrode; measuring means to determine the value of said adjustable set potential and said current flow wherein a variation of said current flow is indicative of corrosion action affecting said test electrodes; display means for displaying said computed potential and said current flowing through said test electrodes.
According to a second aspect of the invention, there is provided, a method of providing bipolar potentiostatic control of test electrodes, said test electrodes forming part of an electrical potentiostatic circuit having an auxiliary electrode and reference electrode wherein said electrodes are immersed in an electrolyte, comprising the steps of applying an adjustable set potential to said potentiostatic circuit; buffering the output of said reference electrode to provide a buffered signal; adding said set potential and said buffered signal to obtain a computed potential; maintaining, by driver means, said test electrodes at said computed potential by controlling the current flow between said test electrodes and said auxiliary electrode; measuring the value of said adjustable set potential and said current flowing through said test electrodes wherein a variation of said current flow is indicative of corrosion action affecting said test electrodes; displaying said adjustable set potential and said current flowing through said test electrodes.